Conventional sinusoidal AC voltage supplies provide only fixed motor speed and are unable to respond quickly to changing load conditions. With the advent of variable frequency drives (VFDs), a better performing motor at lower energy costs can be achieved. VFD driven motors rapidly respond to changing load conditions, for example in response to shock loads. VFD driven motors provide precision torque output and continuous speed control, as well. Because of their many advantages, the utilization of VFDs in industrial applications continues to grow.
A conventional medium voltage VFD driven motor system is described below with reference to FIG. 1. The neutral point N 26 of the DC bus 20 is grounded to protect the transistor switches from potential voltage spikes that would cause insulation degradation and component failure. The heat sink plate of transistors in the inverter bridge is also grounded, however, the ground connection is not shown in FIG. 1. FIG. 2A illustrates grounding of the inverter bridge transistor module via the heat sink plate 126 and is described below in greater detail. Again referencing FIG. 1, three phase cables 30 are connected at one end to the output terminals 52 of the VFD 50. Cables 30 have an inherent capacitance per unit length. The total cable capacitance is shown as CC 32. These cables feed the motor M 40, which also has capacitance due to windings, shown as CM 42, and motor impedance shown as ZM 44.
FIG. 3A is a schematic circuit 300 representation of a conventional VFD driven motor system, for example the drive system shown in FIG. 1. Switch S 60 represents the voltage transitions output from the VFD 50. Upon closing of switch S 60, a voltage transition, from grounded neutral to the positive 22 or negative 23 potential on the DC bus 20, output from the VFD 50 is imposed upon the circuit 300. Ground leakage current IGND 200 flows freely in the ground connection with the voltage transitions due to the motor capacitance CM 42 and the cable capacitance CC 32. Inverter bridge capacitance, CIB 62, is connected from the neutral point N 26 to equipment ground PE 70 and to true earth ground TE 80 in parallel with the short circuit of the neutral point N 26 connection to true earth ground TE 80. In this conventional configuration because CIB 62 is in parallel with the short circuit connection to ground, CIB 62 contributes negligibly to ground leakage current.
Because of their high performance and lower power consumption, VFDs are desirable in a variety of demanding applications, to include fan and pump loads. However, use of VFDs in medium voltage applications can be complicated if low ground leakage current is necessary. Low ground leakage current can be necessary in potentially explosive environments or in environments requiring reduced electromagnetic interference (EMI). High frequency ground leakage currents, up to the MHz range can lead to EMI, for example in radio receivers, computers, bar code systems, and vision systems.
One example of an application requiring low ground leakage current is underground mining; the underground mining environment has unique requirements and safety standards. Underground mining motors are preferably in the medium voltage range (between 690 V and 15 kV) and are typically driven at 4,160 V. A conventional medium voltage VFD providing a 4,160 V output can yield a ground leakage current IGND 200 in excess of ten amps, which flows from the VFD 50 to the motor M 40 in the grounding wire. While using a medium voltage motor facilitates the use of smaller cables, the maximum permitted drive to motor ground wire leakage current IGND 200 can be below 1 Amp.
Unlike conventional AC sinusoidal motor drives, VFDs output voltage transitions on the time order of microseconds. Consequently, large ground leakage currents are induced due to capacitances CM and CC, inherent in a VFD driven motor system, even at relatively low voltages, for example 690 volts. Referring to FIG. 3A, disconnecting the neutral point N 26 of the DC bus 20 from TE ground 80 appears to be a viable means of reducing ground leakage current. A schematic representation of disconnecting the neutral point N 26 from ground in a conventional VFD system 302 is shown in FIG. 3B. As shown in FIG. 3B, disconnecting the neutral point N 26 of the DC bus from TE ground 80 changes the circuit model for ground current leakage current, IGND 202. Inverter bridge capacitance, CIB 62, is now in series with the parallel combination of cable capacitance CC 32 and motor capacitance CM 42. This results in higher impedance for the ground leakage current due to the decrease in total system capacitance. However, disconnection of the neutral point N 26 from TE ground 80 leaves transistors S1-S12 in the inverter bridge susceptible to voltage spikes.
Disconnecting the neutral point N 26 of the DC bus from TE ground 80, leaves the transistors floating relative to the neutral point N 26 of the DC bus. Voltage spikes at full DC bus potential can be applied across the transistors in the inverter bridge. Referring to FIG. 2A, these voltage spikes are transmitted between the transistors' semiconductor substrate 122 and the transistors' heat sink plate 126 across thin insulator 124.
For lower drive voltages, available transistors rated above the difference between the positive and negative DC bus can be employed in a VFD system having the neutral point of the DC bus disconnected from TE ground and left floating. This configuration is successfully employed for example in SMC's Microdrive 2,300 V model1. However, when higher VFD voltage output is needed or desired and when transistors rated at the full DC bus potential are not practical, protecting transistors from full DC bus potential spikes is necessary to prevent reduced component life and component failure. Multiple challenges exist for VFD drive applications. One challenge, for example, is to reduce leakage ground current while protecting the VFD, in particular the inverter bridge. Another challenge is to reduce ground leakage current as much as possible. 1 VFD, Microdrive, 2,300V model, SMC Electrical Products, 2003.
For other applications, the challenge is to provide a reliable VFD system for motors rated at greater than 4160 V. For, example for a motor rated at greater than 4160 V, a VFD providing an output of 6.9 kV output is desirable. However, presently available transistors to build a VFD with a 6.9 kV output are susceptible to compromised transistor insulation and impending component failure. A DC bus rated at 11.5 kV is needed to achieve a VFD output of 6.9 kV. Inverter bridge transistors are available at an insulation rating of 5,100 V. Even when the neutral point of the DC bus is grounded, the transistor module insulation 124 (FIG. 2A) breakdown voltage (5,100 V) is less than half the potential on the DC bus (11.5 kV). Yet another challenge in VFD systems is to protect the inverter bridge comprising available transistors connected in series to provide VFD output voltages greater than 4160 V when transistors are rated at less than half of the DC bus voltage.